High-frequency tetrode with built-in capacitor



Oct. 2, 1951 W W, E|TE| ETAL 2,569,847

HIGH-FREQUENCY TETRODE WITH BUILT-1N CAPACITOR Fired April s, 1949 ssheets-sheet 2 JNVENTORS` WML/,4M IM E/TEL By c/Ac/f A MECU az/GHATTR/VEY Oct. 2, 1951 Filed April a, 1949 W. W. EITEL ETALHIGH-FREQUENCY TETRODE WITH BUILT-IN CAPACITOR `3 Sheets-Sheet 5Screen-Jro-ca+hode 6N ypass capaca''or.

4 8 Anode"\\` @0009@ 2 `/Cai'h'rnde Confrol grid /Screen grid Inpurresona'or Screen grid i'erminal Conrrol grid +erminal Tuning plungerATTORNEY Patented Oct. 2, 1951 HIGH-FREQUENCY TETRODE WITH BUILT-INCAPACITOR William W. Eitel, Woodside, and Jack; Ar. Mc- Cullough,Millbrac, Calif., assignors to-Eitel- McCullough, Inc., San Bruno,Calif. a corporation off California ApplicationApril 8, 1949, S'erialNo.86,231

fa screen gridsuch asatetrcde, on:A account of the higher power gain,and other advantages of thetetrode.V The waytetrodes `have been made `inthe., past i. however, involving; unsuitable electrode terminalarrangements ande requiring meansin the external circuit forbypassingradio frequency currents from-,screen grid tot cathode, it hasnotbeenpossiblefto` utilize'icav-ity circuitry to the `best advantagewith1the tet-rode typetube.

TheV broad object' of our invention is t0 provide an electron` tube andcircuit which` provides `the advantages of a" screengrid tube andalsotheadvantages ot a simple cavity circuit;

Another object is, to. provide` a` tetrode having an internal bypasscapacitor between the screen grid and cathode, incombination withazcoaxial terminal arrangement for the electrodes( Where- 4by the tube,vandassociated circuitry'functionin simpliiied manner.

Still another object istoprovide ar unique;l in- 4 ternal electrodestructure embodying the-fbypass capacitor.

The invention possesses other'objeetsfandafeatures of advantage, some ofwhich; with the foregoing, will be set forth in thafollow-ing"`description of our invention; Itis-,tol` be understood that we do= notlimit` ourselvestc thisdisclosure ofspeciesl of, our invention, as wemay adopt variant embodiments i thereof` within` the scopeof .theclaims.

Referring to the drawings:

i 4 Figure 1 is'avertical sectionalviewroa tetrode embodyingourwinvention; 'and` Figure 2fr shows` a modiiied` tubeconstruction;

Figure 3` is a` diagrammaticsectional viexv of v a4 radio-frequencygenerator embodying our invention.

. Interms of. broad inclusion, our-improved electron tube comprises acathode,` control grid, screengrid and anode, and a bypass capacitor 9Claims. (Cl. S15- 58) wit'nirr` the tubeV envelope and,V connectedtoYthe screen grid and cathode. The preferred con:- struction comprisescoaxial electrodes with=the control grid terminating adjacent the upperend of the cathode, the bypass capacitor. being located above theelectrodes' andiastened toheat isolatingysupports projectingyfrom the`upper ends of thescreen-grid and cathode.` In our improved tubethe-control gridrterrninall1 is at the tube Vend opposite theanode, andvthe,` screen grid terminal is interposed in.. the envelope-Wall betweenthe control grid,terminaland the anode, these grid terminals beingcoaxial with each: other: and with the. anode.. A. radio-frequencygenerator embodyingV our invention,V and. incorporatingr a tube ofthecharacterdescribedicomprises ae plurality of coaxial conductorsexternallyvv of thetube and connected tothe control gridandscreen,gridand anode; the conductors connected-to1the=con `trol grid and screengriddeiining-Aanzinput cavity resonator, and the` conductors: connectedto: the screen grid and anode defining;anoutput-,cayity resonator.

In greater detail and referring-'to` Figure- 1 of the drawings, ourimproved` tetrode comprises,

coaxial electrodes-including a cathode 2, control grid 34 screengrid dand, anodef. Gathodel; is preferably. of the cylindrical indirectlyheated type havinga thermionically active surface: such as theconventional oxide coa-ting, andis preferably heated by radiation froman internal heater coil 'L Grids 3 and Aarepreferably-'of vertical' barconstruction, the. control grid ,3i being openu at the upper end anditerminating adjacent theupper endcf ,cathodef2.. Anode 6. is preferablyof' the externalianodetypeand has a cylindrical recess 8 intoiwhichtheother electrodes project.

Metal anode 6 islocated adjacent the upper end 'of the envelope and,carries the coolerA 5 and also an exhaust tubulation., Ill which=ispinched off `after, evacuation ofthe envelope.

AA tubular stern member, I5;- coaxial with the electrodes. islocatedatthe lower endfof the` en"- velope. This stem member comprises theter"h mnal for the control; grid: 3g the latter being. supported A cna.sleeve .9i projecting from thextubular member 8. Leads. l0 and, H=- for'the heater coil are, brought` out through the stem member,

being. sealed4 through, a. glass disk l2 carried` by a.U-shaped-.sealing ring llbrazed4 at I4.to,the lower` end of'` tubularmember l5.

The tube envelope is made up-in two sections joined. at.a pair ofYinteritting metal rings,- I6 and Il., The upperenvelope-sectionWhichrsup ports theanodc comprises a. cylindricalglass.Wall section I8 sealed at the lower end to a flange I9 on ring I6 and atthe upper end to a sealing ring 2| brazed to anode 6. This sealing ring2| preferably has an upturned lip 2D providing a cylindrical contactsurface which forms the anode terminal. The lower envelope section orheader comprises a glass disk 22 sealed between stem member I and theinner flange 23 of the U- shaped ring I1. VVFinal closure of theenvelope is made by brazing rings I6 and I1 ,together at 24.

United rings I6 and I1 comprise the screen grid terminal ring of ourtube, this terminal being coaxial with the control grid terminal I 5,and interposed in the envelope wall between the anode terminal and thecontrol grid terminal. The

Vscreen grid 4 is supported on its terminal by a cone 26 resting on aflange 21 secured to the inner surface of ring I1. Y

An important feature of our invention is that a bypass capacitor 28 isprovided within the tube envelope for bypassing radio-frequency currentfrom the Vscreen grid 4 to cathode 2. In our improved tube the cathodeis effectively removed from the external R. F. circuits, and no terminalfor R. F. connection to the cathode is shown in the tube structure ofFigure 1, in fact, the entire cathode is supported from the screen grid.

In an ordinary tetrode having terminals for the cathode, anode, controlgrid and screen grid, a common type of amplifier, for example, involvesan input circuit connected to the control grid and cathode terminals andan output circuit connected to the anode and cathode terminals, thescreen grid being connected back to the cathode through an externalbypass condenser. This and other similar arrangements is reasonablysatisfactory for lower frequency applications involving circuitry withlumped constants, but is most awk- Ward to handle at high frequenciesusing coaxial lines or cavities as resonant circuits. The electrodelterminals inherently come out in the wrong relationship for theexternal circuits andthe need forthe external bypass complicates thecircuit problem very badly.

Our improved tetrode is adaptable for use with cavity resonators in avery simple way as shown diagrammatically in Figure 3. The coaxialconductors 3I, 32 and 33 are connected to the control grid terminal,screen grid terminal and anode terminal, respectively, so that the inputresonator formed by the conductors 3I and 32 is connected between thecontrol grid and screen grid, and so that the output resonator isconnected between the screen grid and anode. Since the screen grid .Y isbypassed to the cathode within the tube there is no external bypasscondenser and the cathode is not required to be connected directly tothe external R. F. circuits. All that is required is a D. C. returnconnection for the cathode which may be a simple strap connection 34between the cathode and vone of the heater leads I0 as illustrated inFigures 1 and 3. Suitable D. C. connections as indicated, and suitableinput and output couplings (not shown), are provided as will beunderstood by those skilled in the art.

Referring to Figure l, the bypass capacitor 28 in our improved tube ismounted above the electrodes within the upper region of the anode. Sinceythe control grid 3 is open ended and terminates at the upper'end of thecathode this intermediate grid does not interfere with connecting thecapacitor 28 across the upper ends of the screen grid and cathode. Acapacitor comprising concentric cylindrical metal plates 36 and 31 isShown for purposes of convenient illustration, it

being understood that these condenser plates may be of any othersuitable shape and arrangement such as flat disks extending across thetops of the electrodes. The condenser plates are preferably spaced by asolid dielectric material 38 which also functions to bond the platestogether into a single unitary structure.

The amount of capacitance desired, which will depend upon the electricalcharacteristics of a particular tube, will determine such things asplate size, spacing, number of plates, etc., as will be readilyappreciated. An advantage of using the solid dielectric 38 between thecondenser plates is that close spacings can be achieved which permitsbuilding up a relatively large capacitance in a small physicalstructure. Any suitable high temp-erature dielectric, such as one of therefractory metal oxides, capable of withstanding the operatingtemperatures involved in the tube envelope may be used. This refractoryoxide' insulating material may bev applied as a coating and'sandwichedbetween the plates, and then the condenser structure red in a vacuumfurnace for sintering or bonding the parts together.

In order to thermally isolate the capacitor 28 as much as possible fromthe tube electrodes, and to prevent heat from being conducted away fromthe cathode, the condenser plates are preferably connected to thecathode 2 and screen grid 4 by supporting sleeves 39 and 4I of thinsheet material, preferably of a metal Vor alloy having poor heatconductivity. Holes 48 in the sheet further serve to lower heatconductivity.

By the above arrangement the bypass capacitor 28 is closely coupledelectrically to the cathode -and screen grid, and the capacitorstructure also functions as a support for the cathode. In other words,the cathode as well as the capacitor derives support from the screengrid 4.

If desired, the cathode 2 may be provided with a separate R. F. terminalexternally of the envelope, which may be desired in some circuitapplications. structure in which the cathode is connected at the lowerend by a sleeve 42 to a tubular cathode terminal 43 extending throughthe control grid terminal l5. In this case the latter terminal ispreferably shortened and enlarged to take a sealing ring 44 brazed inplace at 46, the cathode terminal 43 being sealed to ring 44 by a glassbead 41. One of the heater leads I0 is preferably connected directly tothe cathode terminal 43 and the center lead I I is formed as anextension of a prong-like center terminal 48 projecting through thetubular cathode terminal 43. Prong 48 is sealed to the lower end of asleeve 49 by a glass bead 5I which sleeve'projects downwardly intoterminal 43 and is secured thereto at the braze 44.

The modified tube structure is therefore similar to that first describedexcept that it provides an external R. F. terminal 43 for the cathode.The sleeve-like connection 42 to cathode 2 also provides a mount for thelower end of the cathode cylinder, so that the cathode is effectivelysupported at both ends, namely, at the top by the screen grid 4 Via thecapacitor 28 and at the bottom by the cathode terminal 43. Likewise thescreen grid gets support at the top from the cathode by this arrangementsince the result is a mutually supporting structure. This mutual supportfor the electrodes is desirable in some cases, particularly where tubesare subjected to vibra- -tion in service. Also the support at both endsof the electrodes makes it easier to obtain and Figure 2 shows'such amodied tube maintain close and accurate interelectrode spacings.

While we have described our tube particularly in conjunction with cavitytype circuits, it is understood that the improved tube iinds importantuses in applications involving other and more conventional circuitry.

We claim:

1. An electron tube comprising coaxial electrodes including a cathodeand control grid and screen grid, said control grid having an open endterminating adjacent the upper end of the cathode, and a bypasscapacitor of fixed capacitance located above said electrodes andconnected to the upper ends of the screen grid and cathode, saidcapacitor structure being bridged between the screen grid and cathodeabove the terminal end of said control grid.

2. An electron tube comprising coaxial electrodes including a cathodeand control grid and -screen grid, said control grid having an open endterminating adjacent the upper end of the cathode, a bypass capacitor offixed capacitance located above said electrodes, and supports having loWthermal conductivity connecting the capacitor to the upper ends of thescreen grid and cathode, said capacitor structure being bridged betweenthe screen grid and cathode above the terminal end of said control grid.

3. An electron tube comprising coaxial electrodes including acylindrical cathode and control grid and screen grid, said control gridhaving an open end terminating adjacent the upper end of the cathode, abypass capacitor located above said electrodes, and supporting sleevesprojecting from the cathode and screen grid and connected to saidcapacitor.

4. An electron tube comprising an envelope having a recessed anode atthe upper end, a cathode projecting into the anode, a control grid andscreen grid surrounding the cathode, said control grid having an upperend terminating adjacent the upper end of the cathode, and a bypasscapacitor located in the upper region of the anode and connected to thescreen grid and cathode.

5. An electron tube comprising an envelope having a pair of coaxialterminal members, a cathode and control grid and screen grid in theenvelope, the control grid being connected to one of said terminalmembers and the screen grid being connected to the other terminalmember, and a bypass capacitor in the envelope connected to the screengrid and said cathode.

6. An electron tube comprising an envelope having a recessed anode atthe upper end and a tubular stem member at the lower end, a terminalring coaxial with the stem member nterposed in the envelope wall betweensaid stem member and anode, a cathode projecting into the anode, acontrol grid surrounding the cathode and connected to the stem member, ascreen grid surrounding the control grid and connected to the terminalring, said control grid having an open end terminating adjacent theupper end of the cathode, and a bypass capacitor located in the upperregion of the anode and connected to the screen grid and cathode.

7. An electron tube comprising an envelope having a recessed anode atthe upper end and a tubular stem member at the lower end, a terminalring coaxial with the stem member interposed in the envelope Wallbetween said stem member and anode, a cathode projecting into the anode,a control grid surrounding the cathode and connected to the stem member,a screen grid surrounding the control grid and connected to the terminalring, a conductor for the cathode extending through the stem member,.said control grid having an open end terminating adjacent the upper endof the cathode, and a bypass capacitor located in the upper region ofthe anode and connected to the screen grid and cathode.

8. An electron tube comprising an envelope having a recessed anode atthe upper end and a tubular stem member at the lower end, a terminalring coaxial with the stem member interposed in the envelope wallbetween said stem member and anode, a cathode projecting into the anode,a control grid surrounding the cathode and connected to the stem member,a screen grid surrounding the control grid and connected to the terminalring, a heater in the cathode, conductors for the cathode and heaterextending through the stem member, said control grid having an open endterminating adjacent the upper end of the cathode, and a bypasscapacitor located in the upper region of the anode and connected to thescreen grid and cathode.

9. An electron tube comprising an envelope having a recessed anode atthe upper end and a pair of coaxial tubular stern members at the lowerend, a terminal ring coaxial with the stem members interposed in theenvelope wall between the outer stem member and anode, a cathodeprojecting into the anode and connected to the inner stem member, acontrol grid surrounding the cathode and connected to the outer stemmember, a screen grid surrounding the control grid and connected to theterminal ring, said control grid having an open end terminating adjacentthe lupper end of the cathode, and a bypass capacitor located in theupper region of the anode and connected to the screen grid and cathode.

WILLIAM W. EITEL. JACK A. McCULLlOUGH.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 2,256,293 Salzberg Sept. 16, 19412,342,897 Goldstine Feb. 29, 1944 2,396,167 Ferris Mar. 5, 19462,446,379 McArthur Aug. 3, 1948 2,453,148 McCall et al Nov. 9, 19482,455,851 Beggs Dec. 7, 1948 2,489,873 Thorson Nov. 29, 1949

